1. Field of Invention
The present invention relates to a recording liquid adapted for use in inkjet recording, and in particular, to a recording liquid which is useful in forming images on so-called plain paper. Such recording liquid may also be used as a water-based ink composition for water based writing instruments, recording instruments, and pen plotters.
2. Description of the Related Art
A number of investigations have been conducted in recent years on inkjet printers to thereby improve printing properties on a plain paper, and in particular, to improve colorfastness to water and light. As a consequence, inkjet printers and plotters adapted for use with an ink including a pigment for the coloring material are now commercially available. These printers and plotters, however, are associated with the problem of insufficient reliability, for example, clogging, compared to the machine using a dye ink. This problem is particularly notable when a highly permeable solvent is added to the ink for controlling wettability, and the pigment often became aggregated as a result of change in the nature of the dispersant.
In view of such situation, U.S. Pat. No. 5,571,311 and Japanese Patent Application Laid-Open JP-A) No. 10-510862 propose an ink having improved dispersibility, stability after dispersion, and reliability by surface treating a carbon black with a compound having diazonium group, and introducing carboxylic acid and sulfonic acid as a functional group. U.S. Pat. No. 5,922,118 discloses an organic pigment for use in a color ink wherein the carbon black has been surface treated and introduced with carboxyl group and sulfonyl group as in the case of U.S. Pat. No. 5,571,311 or the like. JP-A No. 10-273606 discloses an organic pigment which has been oxidized by using sulfamic acid, and JP-A No. 11-246807 discloses an organic pigment wherein sulfonic group has been introduced by Friedel-Crafts reaction by using a sulfonating agent.
The pH of a water soluble ink is generally adjusted for the purpose of preventing clogging of the nozzle and corrosion of the members that come in contact with the ink. The surface treated pigments as described above, however, are associated with the problem that they change the pH, in particular, lower the pH when the pigments are exposed to the change in temperature and other environmental factors or as the time lapses despite their improved dispersibility and stability after the dispersion. Such change in the pH resulted in the corrosion and erosion of various constituent members including the head having an ink supplying means and a liquid chamber including members comprising silicon or glass and/or metal such as nickel, SUS, or the like, and this, in turn, adversely affected the ejection properties.
In view of the situation as described above, JP-A No. 04-226175 discloses an ink wherein a sodium borate, N-tris(hydroxy)-methyl-2-aminoethanesulfonic acid, N,N-bis(hydroxylethyl)glycine, triethanolamine or tris(hydroxymethyl)aminoethane has been added as a pH buffer with the particular dye for the purpose of stabilizing the pH of the ink. JP-A No. 08-41398 discloses an ink containing at least one compound selected from 2-(cyclohexyl-amino)ethane-sulfonic acid, 3-N-cyclohexylamino-2-hydroxypropanesulfonic acid, 3-N-cyclohexylaminopropane-sulfonic acid, and glycine with an alkaline metal hydroxide or carbonate, wherein pH has been adjusted to the range of 9.5 to 9.8, and which exhibits small pH decrease after storing at 60° C. for 1 week. JP-A No. 09-268267 discloses an ink containing a dye and 2-amino-1,3-propanediol derivative as an organic weak base, wherein the pH has been adjusted to the range of 8 to 10. Since these inks contain dyes having buffering capability, the pH can be kept in the range of 9 to 9.3 even after storing at 60° C.
With regard to the pigment-based ink, JP-A No. 04-149286 discloses a pigment-based ink using a dispersant wherein a carbon black having a pH of up to 6 is used with a buffer to adjust the pH to the range of higher than 6 to less than 8. In the case of a pigment-based ink, pigment often aggregates at a high ion strength, and therefore the pH adjusters used in the dye-based inks as described above cannot simply be applied to the pigment ink, and such application is particularly difficult in the case of an ink employing a surface treated pigment.